The invention relates to cathodic protection (CP) systems and, more specifically, to a method for designing CP systems, to a method and apparatus for mapping the distribution of CP current on concrete structures and to an xe2x80x9cexpertxe2x80x9d cathodic protection system.
Cathodic protection is a popular technique that is commonly used to minimize corrosion of metals in a wide variety of large structures including, bridges, pavements, parking lots and pipelines. This technique is based on the principles of electrode kinetics, which can be briefly described as follows. In the absence of any polarization, a metal in contact with concrete or an electrolyte will remain at its corrosion potential (Ecor). At this potential, the metal surface sustains at least two reactions occurring at equal rates: a metal dissolution (or anodic metal oxidation) reaction, and a cathodic conjugate reaction, such as oxygen reduction or hydrogen evolution.
If the metal is electrically polarized to potentials positive to Ecor, the metal dissolution reaction will be accelerated, while the cathodic conjugate reaction will be decelerated. The converse is true when the metal is polarized negative to Ecor. Thus, when the metal is polarized away from Ecor to a positive or negative value, a net anodic or a net cathodic current, respectively, will flow across the metal/electrolyte interface. A metal is said to be under cathodic protection (CP) when it is polarized sufficiently negative to Ecor to reduce the metal dissolution rate by three orders of magnitude or more. Under most conditions, a polarization of about xe2x88x92200 to xe2x88x92300 mV is sufficient to achieve cathodic protection.
Excessive cathodic polarization should be avoided to prevent onset of hydrogen evolution reaction, and to reduce the possibility of hydrogen embrittlement of the metal. Furthermore, cathodic polarization, like corrosion, is a surface process. Therefore, to achieve uniform protection at all locations on a given surface, it is imperative that the cathodic current density is uniform at all locations. Any nonuniformity in the current flow, especially with values less than some critical minimum, can cause localized variations in the metal dissolution rate. This can result in the structure corroding more severely in some places than in others. In a bridge, for example, if the CP current is nonuniformly distributed, those parts of the bridge that do not receive the current will continue to corrode, while those that do receive CP current will be well-protected from corrosion.
In typical CP systems used in protecting metal-concrete structures, the metal is usually steel, and the cement and water form the electrolytic medium. Generally, the CP system has a rectifier as the voltage source. The return electrode for the current is either a palladium-coated titanium mesh, a thin layer of zinc, or a conducting polymer mixed with concrete. They are xe2x80x9cinertxe2x80x9d electrodes, not consumed or destroyed by the reactions associated with the cathodic protection, and are called ground beds. Generally, the ground bed is two-dimensional, is spread over the entire structure, and is covered with concrete and asphalt.
In concrete structures, CP is used to protect the reinforcing steel bars, commonly referred to as rebars, from corrosion. On bridge structures, CP has traditionally been used to protect only the deck sections, but recently, support structures have also been protected. In general, all the rebars are electrically connected to one another, and the electrical connections between the rebars and the rectifier are made at one or two remote locations on the bridge. Similarly, the electrical connections between the ground-bed and the rectifier are also made at one or two remote locations. Thus, in most cases, the ground-bed is distributed evenly with respect to the rebars; however, the electrical contact points are highly localized.
During the past 30 years, federal and state highway administrations have been testing and assessing the merits of the CP technique. As a part of that initiative, over 350 bridges in North America are now under CP. Most of these bridges are located in a wide variety of geographical locations, from Washington to Maryland, and Florida to New York. Several more bridges in Canada, with approximately 60 in the Quebec Province alone, are also under CP. Thus, intended or not, they are exposed to a wide variety of environmental and climatic conditions.
If the past 30 years of history were considered as the first step toward exploring CP as a viable technique to protect bridges from corrosion, then the results from those studies are somewhat mixed at best. During the past 10 years, there has been a profusion of reports on the status of CP in several state and federal bridges. These reports suggest that cathodic protection has had mixed success and appears to be more effective on some bridges, as compared to others. The causes of failures have been attributed to a variety of reasons, ranging from inadvertent shutdown of the rectifiers to improper electrical connections. Studies correlating climatic and environmental factors to the effectiveness of CP have yet to appear in any published literature. In essence, the CP technique for concrete structures is still evolving.
The future of cathodic protection (CP) as applied to rebars in concrete bridges is strongly dependent upon the design of CP systems. The design, in turn, determines the effectiveness of CP in minimizing corrosion and the cost of implementation and maintenance. The major drawback of contemporary designs has been excessive flow of CP current in some parts of the bridge, and little or no current in-others.
The use of sensors to manage CP systems in concrete structures is also evolving. Earlier designs used potential-measuring sensors, such as reference half-cells, to monitor the level of electrochemical potential drop across the rebar/concrete interface. In practice, these sensors can only be kept at a finite distance from the interface; hence, a large resistive drop due to the resistance of the concrete is always included as part of the measured potential. This condition has posed serious limitations on both monitoring and maintaining appropriate levels of CP for the rebars. Besides, using potential to monitor CP systems requires measurements to be made at short intervals, i.e., 4 ft or less. This situation could mean installation of virtually hundreds of reference electrodes, especially for large structures such as bridges.
The installation cost of cathodic protection systems on a bridge is reportedly in the range of 15% of the cost of the construction of the bridge. In addition, the recurring expenses due to maintenance and management of CP over its lifetime can increase its cost by several fold. Therefore, in the future, the use of CP techniques is likely to be determined as much by cost considerations, as by its technical merits. There is a need for options to reduce design and installation costs, as well as maintenance and management costs.
The primary goal of the invention is to achieve a uniform distribution of current over an entire structure at all times. To achieve such uniform distribution, the invention provides for an xe2x80x9cexpertxe2x80x9d CP system controlled by a variety of current and environmental sensors and a dedicated microprocessor. The invention also includes: (1) development of numerical techniques that can be used in CP system designs; (2) remote sensing and mapping of the distribution of CP currents in bridges; and (3) correlating the effect of micro-climatic changes to the distribution of CP currents.
The invention uses numerical techniques such as the finite element method (FEM) to model the current and voltage distribution in concrete. The geometric arrangement of ground-beds and the ideal locations for the electrical contacts vis-a-vis the geometry of the bridge and the rebars can thereby be predicted.
The current mapping aspect of the invention uses a magnetic sensor to sense the magnetic field generated by the CP current, and a voltmeter or an oscilloscope to measure the output of the magnetic sensor. The invention also uses a current interrupter to interrupt the CP current at the source.
The current is mapped by placing the magnetic sensor on or above the concrete surface. The current is also mapped by burying the magnetic sensor inside the concrete. The sensor is surrounded by air, concrete, water or other solids or fluids during the measurement. By moving the sensor from one location to another, the current is mapped over the entire structure. Unlike monitoring the potential, as discussed above, mapping the current does not involve errors from resistive drops, and, as described, only a very few sensors are needed to monitor CP.